1. Field of the Invention
This invention relates to a private branch exchange system and, more particularly, to a private branch exchange system which accommodates external line wires and extensions and, on the extension side, communications equipment usable in event of a power failure, wherein communications such as a telephone conversation can be carried out by the communications equipment for power failure when a power failure occurs.
2. Description of the Prior Art
In a private branch exchange, a well-known technique in the prior art is to change over the circuit to a telephone for power failure when a power failure occurs, thereby making it possible to receive calls and make calls during the power failure.
FIG. 4 illustrates the construction of the conventional private branch exchange apparatus. As shown in FIG. 4, numeral 400 denotes an external line wire, 401 a line-wire changeover relay, 404 an exchange apparatus, 406 a extension changeover relay, and 408 an extension telephone usable in the event of a power failure. When the exchange 404 is being supplied with power from a commercial power supply or the like in a normal manner, the line wire 400 is connected to the exchange 404 via the make side (indicated by the dashed line) of the line-wire changeover relay 401 and a line 403.
A telephone call is placed by way of a path leading to the power-failure backup telephone 408 via a line 405, a line on the make side (indicated by the dashed line) of the extension changeover relay 406, and a line 407. When the supply of power to the exchange 404 ceases owing to a power failure or the like, the line-wire changeover relay 401 and the extension changeover relay 406 are restored automatically so that the line wire 400 is connected directly to the power-failure backup telephone 408.
When supply of power to the exchange 404 is restored, the line-wire exchange relay 401 and extension changeover relay 406 are driven automatically so that a telephone call can be placed via the ordinary route, described above.
Further, in another example of the prior art, the line-wire exchange relay 401 and extension changeover relay 406 may be replaced by manually operated changeover switches.
A problem encountered in the above-described example of the prior art is that when the extension telephone usable in a power failure starts to be used during a power failure and then power is restored during a telephone conversation, the path being used for the conversation is cut unconditionally if the line-wire and extension changeover means are automatic means, such as relays. As a result, the call is cut off while still in progress.
If the line-wire and extension changeover means are manual means such as switches, on the other hand, an operating error is possible if the switches are operated erroneously or if the operator forgets to make a changeover. In addition, operating the switches is troublesome and therefore the system is not easy to use.